1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a facsimile or a printer and, more particularly, to an image forming apparatus capable of switching a fixing speed in accordance with the kind of a sheet.
2. Description of the Related Art
In the field of the image forming apparatus such as the copying machine, the market of recent years demand the apparatus for matching various materials such as cardboard or coated paper, and the current situation is that many products can match.
Generally, more calories than that for the plain paper are required for fixing an image to a sheet having a basis weight of 300 g/m2, such as “cardboard” or “super cardboard”.
In case the coated paper has a large basis weight, moreover, more calories than that for the plain paper are also required not only for the coated paper but also for the sheet of a high gloss, which is frequently demanded in recent years.
In case these sheets are to be fixed, therefore, the fixing speed is made lower than that for the plain paper. The more calories are applied to the sheets by elongating the time period, for which the sheets pass through the fixing unit.
As the method for lowering the fixing speed, there are a method, by which only the fixing speed is decelerated while the transferring speed being left the same as that for the plain paper, and a method, by which both the transferring speed and the fixing speed are likewise decelerated. Many copying machines have been produced according to the individual methods.
There are also products, which are enabled, although a few, to match the board paper by widening the spacing the sheets passing through the fixing unit than that of the plain paper thereby to store and restore the calorie of the fixing unit, without decreasing the fixing speed.
An example of the copying machine, which decelerates only the fixing speed while the transferring speed being left equal to that for the plain paper, is shown in FIGS. 8A to 8E.
FIG. 8A shows a portion of an image forming apparatus having four image formation units. In this image forming apparatus, the sheet is conveyed, while being absorbed on a transfer belt 150, to the individual transfer units. After an image on a photosensitive drum of the fourth image formation unit 101 was transferred to the sheet by a transfer unit 105, the sheet is conveyed to a fixing unit 109 by a conveyor portion 160. The toner on the sheet is heat-fixed by the fixing unit 109.
The detailed actions will be described in the following. In FIG. 8B, the image is being transferred to the sheet S1 at the fourth image formation unit 101. At this time, the conveying speed of the sheet S1 by the transfer belt 150 and the conveyor portion 160 naturally takes a value V1 equal to the transferring speed.
Next, when the sheet S1 moves in its entirety to the conveyor portion 160, as shown in FIG. 8C, the conveyor portion 160 is decelerated to the fixing speed. The conveying speed (=the sheet moving speed) of the conveyor portion 160 and the fixing unit is V2.
The sheet S1 is fixed as it is at the speed V2 (<V1) (FIG. 8D). After the sheet trailing end passed the conveyor portion 160, moreover, the conveying speed of the conveyor portion 160 is accelerated to V1 and is prepared for the conveyance of a next sheet S2 (FIG. 8E).
In the case of this control, the spacing of the sheets in the image formation unit has to be set to the sum of the sheet size and the margin of the control, the estimated dispersion of the sheet position and the time period necessary for the speed change.
The sheet spacing of the case of the plain paper is about 50 to 100 mm.
The sheet spacing of the case of the plain paper is about 50 to 100 mm. On the other hand, in case the aforementioned control is made on the sheet of A3 size (of 420 mm in the conveying direction), the sheet spacing in the image formation unit has to be set to at least about 450 mm, considering the extras. Here, the succeeding sheet chases the preceding sheet at the speed V1 so that the sheet spacing in the fixing unit cannot be narrowed over 200 to 300 mm at most, depending upon the difference between the speeds V1 and V2.
In short, in the aforementioned control, the image forming efficiency is deteriorated for the causes that the fixing speed is lowered and that the sheet spacing is widened.
Here will be described the method for decelerating the transferring speed and the fixing speed equally. In this case, the image transfer unit also has to convey the sheet at the same speed as the fixing speed.
Therefore, the distance of the conveyance at the fixing speed, i.e., at a speed lower than that of the plain paper is elongated to elongate the image forming time period necessary for forming the image of the first sheet, i.e., the first copy time period (Fcot). Especially in the case of an intermediate transfer type copying machine, the extremely elongation of the distance from the first image formation unit to the transfer unit raises a serious problem.
Moreover, not only the transfer unit but also the photosensitive drum has to be driven at the transferring speed and the fixing speed so that the time period needed therefor is elongated. In addition, the latent image may diffuse during the time period from the exposure to the development, and the affections of the vibrations of the photosensitive drum drive may rise to deteriorate the image.
By making the spacing of the sheets to pass through the fixing unit wider than that of the plain paper without changing the fixing speed, moreover, the method for storing and restoring the calories of the fixing unit is higher in the temperature drop of the fixing unit or fixing roller at the time of passing the sheet than that of the method for decelerating the fixing speed. Then, the fixing properties are different between the leading end and the trailing end of the sheet thereby to raise such a problem in the image quality that the gloss of the image may be different between the leading end and the trailing end.